Puedes usar los dos, pero tienen significados distintos.
Estaba en un restaurante = I was in a restaurant
Estuve en un restaurante = I was in a restaurant
What's the difference?
1. You use the imperfect for background information, and you use the preterit for actions that happen.
Estaba en un restaurante cuando vi la explosión en el estacionamiento = I was in a restaurant when I saw the explosion in the parking lot
Estuve en un restaurante = I was in a restaurant
2. You also use the imperfect for things that may or may not be already finished at the present time, and preterit for things that you know are finished.
Estaba en un restaurante = I was in a restaurant (but you could still be in the restaurant for all we know)
Estuve en un restaurante = I was in a restaurant (but you are not anymore)
Hope that helps!